There are three predominant methods used for the modeling of ships. First a detailed section of a ship can be represented. In this method, essentially, a “slice” out of the ship is modeled in detail and loads and boundary conditions are applied to the ship structure and/or keel surrounding this detailed section of the ship. The primary disadvantage of this method is that the entire ship is not represented in one activation of the model. Therefore, gross ship motions cannot be represented and the ship-wide mass and stiffness are not accounted for. This can lead to incorrect representations in the frequency spectrum.
The second modeling method represents the ship as a beam. This method works fairly well in obtaining gross motions since most ships are significantly longer than they are wide, and thus resemble a beam from a mathematical standpoint. The advantages of this method are that gross ship motions are represented quite well and the ship's actual mass and stiffness can be accounted for leading to good representations in the frequency spectrum. The primary disadvantage of this method is that fine details of the ship cannot be represented. Thus, internal ship spaces and equipment cannot be represented. The way that the beam model is connected to the ship's hull is through a series of stiff connections from the hull to the main beam model of the ship. This series of stiff connections or webs is used to transfer the applied loads from the hull representation to the beam model. This web technique is a fairly typical approach for transferring hull loads to the beam model. Additionally, since the desire has been to represent the internal ship spaces and equipment, the beam method is not sufficient and the ship section does not provide the appropriate ship motions to the modeling and simulation.
The final approach has been to extend the detailed “slice” of the ship to the entire ship, essentially creating a ship model with every ship space contained in the model. This has the significant disadvantage of resulting in mathematical models that cannot be solved on most currently commonly available computer systems. Other disadvantages are the significant amounts of time that are required to build the mathematical model and the fact that shipboard frequencies tend to be “under predicted” using this method.